1. Field of the Invention
The present invention relates to a flexible printed circuit (FPC), and more particularly, to an FPC to minimize vibration energy of noise.
2. Description of the Related Art
A hard disk drive is an auxiliary memory device of a computer to write and read information on a magnetic disk by using a magnetic head operating as an electromagnet.
FIG. 1 illustrates the structure of a conventional hard disk drive 20. Referring to FIG. 1, the conventional hard disk drive comprises: a disk 13 to store information; a spindle motor 15 to rotate the disk 13; an actuator 23 having a magnetic head 25 to write and reproduce information on and from the disk 13; a voice coil motor 21 to drive the actuator 23; a flexible printed circuit (FPC) 11 to transfer electrical signals from a printed circuit board (PCB) (not shown) to the actuator 23; and a bracket 31 to support the FPC 11.
The disk 13 is formed of a parking zone micro-processed by a laser at an inner region of the disk 13 and a data zone at the outer circumference of the parking zone, on which magnetic signals are written. In this case, the parking zone is coupled with the spindle motor 15 to mount the magnetic head 25 in a power-off state. Servo signals to report the location of information to be written are pre-written on tens of thousands of tracks along the circular shape of the disk 13.
The actuator 23 includes a fantail unit 19 having the voice coil motor 21 to drive the actuator 23; a pivot bearing 17 at the center of rotation of the actuator 23; and the magnetic head 25 having a write head to write data on the disk 13 and a read head to reproduce information from the disk 13.
The PCB transfers electrical signals to the FPC 11 and the FPC 11 transfers the electrical signals to the actuator 23. The electrical signals are transferred from the actuator 23 to the voice coil motor 21. The actuator 23 is rotated centering upon the pivot bearing 17 by electromagnetic force from interaction between current and magnets in the voice coil motor 21. Accordingly, the actuator 23 moves from the parking zone to the data zone.
FIG. 2 illustrates the structure of the conventional FPC 11. Referring to FIG. 2, head signal patterns RDX; Read Data X, RDY; Read Data Y, WDX; Write Data X, and WDY; Write Data Y to transfer and/or receive the electrical signals to and from the magnetic head through a pre-amplifier (not shown), voltages VDD; positive DC supply and VSS; negative DC supply to supply voltage to the pre-amplifier, a ground GND to ground, and a current pattern VC; Voice coil to apply the current to the voice coil motor 21 are arranged on the conventional FPC 11.
In the case of writing and/or reading data on the hard disk drive 20, the magnetic head 25 moves along the tracks of the disk 13 to write and/or read data. It is most preferable that the rotating track of the disk 13 forms an exact circle, and the magnetic head 25 precisely writes and reads data. However, in reality, the disk 13 rotates in a distorted circle caused by vibration of the spindle motor 15. Accordingly, a runout occurs, in which there is a difference between the locations of the track and the magnetic head.
Two kinds of runout include repeatable runout (RRO), which repeats every rotation and non-repeatable runout (NRRO), which does not repeat every rotation.
The RRO means the rotation of the disk 13 to form a waveform with a specific period caused by the assembly deflection of the disk 13 or the vibration of the hard disk drive 20. Since the waveform is repeated periodically, differences in location between the track and the magnetic head 25 can be compensated for by recording waveform information in a servo memory.
In order to compensate for the RRO and test reliability, a burn-in test is performed during the manufacturing process of the hard disk drive by varying conditions such as temperature and voltage. However, it is difficult to completely compensate for the RRO because various factors like FPC bias, windage bias, and heat complicatedly operate in the manufacturing process. The FPC bias means the force applied from the FPC to a head stack assembly (HSA) mechanically. The windage bias means the force applied to the HSA according to the effect from rotating fluctuation of the disk to the FPC.
In the FPC 11, since the head signal patterns RDX, RDY, WDX, and WDY are physically close to the current pattern VC and the voltage patterns VDD and VSS, through which a relatively large current flows, noise affects the head signal patterns RDX, RDY, WDX, and WDY. In addition, a low frequency of about 340 Hz caused by the natural frequency of the FPC 11 affects servo control.
In particular, since the write head signal patterns WDX and WDY are nearer to the voltage patterns VDD and VSS and the current pattern VC than the read head signal patterns RDX and RDY, the effects are much more serious in a write mode than in a read mode.
Consequently, since the compensation values for the RRO vary according to the operating environment and the kind of hard disk drive, the performance of hard disk drives is affected by disturbances like NRRO and low frequencies of under 1 kHz, which are not servo control frequencies of the FPC. NRRO indicates the rotation of the disk to form a distorted waveform without a specific period.
Referring to FIG. 3, a frequency response graph of the conventional FPC illustrates current strength peaks at around 340 Hz and 750 Hz. Moreover, the peaks are repeated at radio frequencies above 1 kHz. Since the natural frequencies of the conventional FPC are located at the frequency bands 340 Hz and 750 Hz, resonance occurs between the natural frequencies of the FPC and noise. Consequently, the vibration energy at the frequency bands is relatively strong, thereby generating the peaks.
As described above, in a conventional hard disk drive, seek time, which means the time to seek a track, and position error signal (PES) are increased by the noise effect from the current pattern VC and the voltage patterns VDD and VSS, by the difference between compensation values of the RRO, and by the effect from the low frequency noise. As a result, the performance and yield rate of the conventional hard disk drive is deteriorated.